Note: Descriptions are shown in the official language in which they were submitted.
1C~70583
This invention relates to an improved solar heater
assembly for heating air or other gases having increased
efficiency in the transfer of solar energy falling on the
assembly to a gas stream and to an improved heat-exchange
plate used in the assembly.
With the decreasing supply and consequently high
cost of conventional fuels, increased attention is being
given to solar energy as a substitute, at least in part, for
other fuel sources for use in space heating homes and other
structures. Many of the solar energy generators proposed
for use in such applications are similar in their principle
of operation. Typically, such a solar energy generator
comprises a sheet of glass or other material which is trans-
parent to the heat rays of the sun and which forms one side
of a conduit for fluid flow. Below the transparent sheet,
within the conduit, there is positioned a heat-conducting
sheet of plate having a coating of a suitable dark material
which absorbs the solar radiant energy which passes through
the transparent sheet and converts the radiant energy to
heat, causing the temperature of the plate to rise. A
fluid, typically air or other gas, is passed through the
conduit and i8 heated by contact with the energy-absorbing
plate within the assembly. The heated stream of fluid which
leaves the generator is then used in appropriate conventional
fashion for heating a home or other building, or grain
conditioning (corn drying, dehydrating alfalfa, etc.3.
The suitability of a solar heat generator of this
type as a su~stitute for other energy sources depends on the
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1~70583
efficiency with which the solar energy is collected and
transferred to the stream of f~uid, and also on the amount
of power necessary to keep the stream of fluid moving
through the generator. It is, cf course, desirable to
maximize the rate of heat transfer from the energy-absorbing
plate to the fluid stream while minimizing the amount of
power required to pass the fluid through the solar heater.
The design of a solar heat generator which will achieve
simultaneously both of these objectives presents a problem,
however, since the conditions which tend to increase one
effect tend to reduce the other. Thus, in order to maximize
the rate of heat transfer between the energy-absorbing plate
and the stream of fluid, the area available for heat transfer
from the plate to the fluid should be relatively high. An
increase in heat transfer area can be achieved in conventional
fashion by the use of heat-conducting fins projecting from
the surface of the heat-absorbing plate which contacts the
fluid stream. While the use of fins produces a desirable
increase in the area for heat transfer, however, such use
has another effect which tends to reduce the overall heat
transfer rate. This undesirable effect arises from the fact
that when a fluid such as a gas flows generally parallel to
and in contact with a stationary surface, such as a fin,
there is always present at the interface between the surface
and the gas stream a relatively stagnant boundary layer of
gas which acts as insulation and tends to decrease the rate
of heat transfer from the fin to the main body of gas. This
stagnant boundary film layer tends to increase in thickness
along the length of the fin in the direction of gas flow,
and thus the insulating effect is greater at the downstream
end of a fin than it is at the upstream end. Another dis-
~070583
advantage arising from the use of fins is that the friction between the gas
stream ana the fins increases the amount of power necessary to keep the stream
of gas in motion at a given rate of flow. While the insulating effect of the
boundary film layer can be reduced by limiting the length of the fins and
arranging them in a staggered fashion in order to maintain a constant finned
area, a staggered arrangement tends further to increase the frictional forces
and hence the amount of power required for moving the gas stream.
In accordance with the invention, there is provided a heat exchange
plate adapted for heat transfer to a stream of fluid moving in a direction
generally parallel to and in contact with a face of said plate, comprising:
a generally flat heat-conducting base plate~ a plurality of flat thin heat-
conducting fins attached to and extending outwardly from said face of said
base plate; said fins being arranged in a plurality of adjacent rows trans-
verse to the direction of fluid flow, all of the fins in a row being
generally parallel to the direction of fluid flow, the lateral spacing
between adjacent fins in all rows being substantially constant, each row
of fins being laterally d~splaced in one direction from the adjacent upstream
rowr the lateral spacing between adjacent fins in a row being S, the length
of the flns heing about 0.75 S to 5.0 S, the height of the fins being about
1.0 S to 5.0 S, and the lateral displacement between fins in adjacent rows
being about 0.1 S to 0.5 S.
m e invention will be better understood from the following detailed
aescription thereof, taken in conjunction with the accompanying drawings,
in which:
Figure 1 i6 an isometric view of the heat exchange plate used in the
invention, showing the fins extending outwardly from one side of the plate
in staggered fashion;
Figure 2 is a ~op view o~ the plate of Figure 1,
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with portions of similar plates arranged at the top and left
edges of the plate to form a larger composite plate structure;
Figure 3 is an isometric view in schematic form of
a typical solar air heater assembly employing the heat
exchange plate structure of Figure 2;
Figure 4 is a sectional view along the line 4-4 of
Figure 3 showing the heat exchange plate in position in the
solar air heater assembly; and
Figure 5 is an enlarged detail of a section of the
plate shown in Figure 2.
As shown in Figures 1 and 2, the invention comprises
a heat exchange plate 10 having a flat base plate 11, to one
surface of which are affixed a plurality of upstanding fins
12. Both base plate 11 and fins 12 are made of a thin,
heat-conductive material, such as aluminum or other metal,
the surface of which is adapted (e.g., polished) to aid in
the transfer of heat from the plate to a stream of gas in
contact therewith, in accordance with principles known to
those skilled in the art. In the embodiment shown, each of
fins 12 is square or rectangular, although it is also within
the scope of the invention to use fins having other shapes,
e.g., trapezoidal or arcuate. It should be understood when
any dimension of a fin, such as its length, is referred to
herein, the average dimension is intended.
As shown in Figure 2, the fins 12 are arranged in
rows extending transversely and preferably perpendicularly
to the direction of air flow. While it is preferred that
each of fins 12 be perpendicular to base plate 11, i.e.,
that the fin form an angle of about 90~ with the base plate,
it is also within the contemplation of the invention to use
angles of attachment which differ from 90~. Al~ of the fins
1070583
in a row are generally parallel to the direction of air flow
and equidistantly spaced along the row. m e fins in any
given row (e.g., 12, Figure 5) are laterally displaced a
constant distance D from the fins (e.g., 12a, Figure S) in the
adjacent upstream row. The direction of displacement
(i.e., right or left) is constant throughout the plate.
While the actual dimensions of the fins 12 used in
a specific heat exchange plate 10 of the invention depend on
the particular requirements, e.g., the heat load, the
desired quantity of air to be heated, and the temperature to
be achieved in the air stream, the relative dimensions of
the fins and the spacings of the fins in the plate can be specified.
For an arrangement in which the spacing between
adjacent fins 12 is S (Figure 5), the length L (Figure 5) of the
fins i8 about 0.75 S to 5.0 S, and preferably about 1.0 S to
3.0 S, and the height H (~igure 1) of the fin is about 1.0 S
to 5.0 S and preferably about 1.375 S to 4.13 S, while the
lateral displace~ent D (~igure 5) between the fins in one row
and the fins in the next adjacent row in the direction of
air travel is about 0.1 S and 0.5 S and preferably within the
range of about 0.125 S to 0.375 S and S has a value of
about 0.75 to 1.25 inches.
In the preferred form of the invention shown in
the figures, it will be seen that there is no overlap,
in the direction of air travel, between the end of the fins
in one row ana the beginning of the fins in the downstream
row, i.e., the projected ends of the fins in adjacent rows
fall on a line. It is also contemplated, however, that an
overlap of up to 0.1 S or a gap of up to 0.1 S between the
ends of the fins in adjacent rows can also be used.
As seen in Figure 1, two adjoining edges of base
plate 11 are provided with offset lips 13 and 14, which
~07(~5~3
permit a plurality of individual heat exchange plates 10 to
be assembled with a free edge of base plate 11 resting on
and supported by the lips 13 and 14 of adjacent plate
assemblies. In this manner, a composite heat exchange
surface can be made to any desired size by the use of
individual plates such as that shown in Figure 1. A portion
of such an assembly is shown in Figure 2, with each plate in
turn supporting and being supported by the lips 13 and 14 on
the individual plates. In such a construction, any suitable
means of attachment between adjacent plates can be used,
such as rivets, soldering, welding and the like.
As previously indicated, although the actual
dimensions of the fins used in particular embodiments of the
invention may vary, depending on the specific requirements
of a particular use, the benefits of the invention will be
obtained provided that the disclosed relative proportions in
the height, spacing and displacement of the fins are observed.
The greatest use of the invention will probably be found in
space heating and cooling, grain conditioning, and the like.
For the sizes of heat generator which will generally be
useful for such purposes, it has been found that the spacing
S between fins suitably on the order of 1 inch, with the
other dimensions being in the proportions previously given.
A typical solar heat generator in accordance with
the invention which is suitable for use in space heating and
cooling of homes is illustrated in Figures 3 and 4. As
shown, the generator comprises a ~rame consisting of side
walls 21 and bottom 22, attached to side support members 23,
all of which are suitably made of wood or other heat insu-
lating material. At the top of the structure and spanning
the distance between side walls 21 is a sheet 24 of glass,
plastic, or other material which is transparent to solar
1070583
energy. Sheet 24 is supported at its outer edges by insu-
lating spacers 26 and held by conventional fasteners 27. As
an optional feature which may be desirable for use of the
inv~ntion in relatively cold climates, a second sheet 28 of
transparent glass or plastic can be mounted a short distance
below glass sheet 24. The layer of air between sheets 24
and 28 acts as an insulating zone to prevent the escape of
heat from within the heat generator.
Mounted a short distance below sheet 28 is a heat
exchange assembly such as that shown in Figure 2, with the
base plate 11 uppermost and fins 21 depending therefrom.
The upper surface 31 of the asembly, i.e., the surface on
which radiant energy passing through plates 24 and 28 will
impinge, is preferably treated to facilitate the absorption
of the radiant energy, as by coating with a flat black paint
or other suitable material in a manner which is known to
those skilled in the art. Disposed immediately below the
lower edges of fins 21 is a layer 29 of a suitable heat-
insulating material, such as glass fibre mats, used to
prevent the escape of heat from the generator.
The assembly of Figure 4 provides a conduit for
air flow bounded on the top by sheet 24 (or, optionally, by
sheet 28), on the bottom by the upper surface of insulation
29 and on opposite sides by insulating supports 23 and
spacers 26. The conduit is suitably equipped with con-
ventional inlet and outlet flow control means (not shown)
for controlling the air flow. The air to be heated is
introduced into the conduit in any appropriate manner and in
passing therethrough comes into contact with the heat
exchange plate assembly, which transfers the heat generated
by absorption of the solar radiant energy to the stream of
1~17~)583
air, as indicated schematically in Figure 3.
A typical assembly of the type illustrated in
Figures 3 and 4 has a dimension of about 7 inches between
glass plate 24 and the bottom 22 of the assembly; a spacing
of about 3/4 inch between plates 24 and 28 and between plate
2g and the top surface of heat exchange plate lO; fins 21
having a length and width of about l inch; a displacement
distance between rows of fins of about 1/4 inch; the width
of the assembly being about 4 feet and the length about
8 feet. In the operation of such a heat generator, the
efficiency of heat transfer from the plate to the stream of
air is such that the bulk of air passing therethrough is
heated to a temperature which approaches within about 10F
the temperature of the heat transfer plate. By comparison,
under essentially the same conditions, but without the use
of the fins in the heat exchange plate, the differential
between the temperature of the plate and the air exceeds
about 40F. This increase in efficiency of heat transfer is
achieved without any significant increase in power require-
ment.
